1. Field of the Invention
The present invention relates to an EL (electro luminescence) display device having a semiconductor element (an element using a semiconductor thin film) formed on a substrate, and electric equipment using the EL display device as a display (display portion). The EL (electroluminescent) devices referred to in this specification include triplet-based light emission devices and/or singlet-based light emission device, for example.
2. Description of the Related Art
In recent years, a technique by which a TFT is formed on a substrate has greatly progressed, and the application of that technique to an active matrix display device has been increasingly developed. In particular, a TFT using a polysilicon film enables high-speed operation since it is higher in field effect mobility (also called mobility) than a conventional TFT using an amorphous silicon film.
Attention has been paid to the above active matrix display device because various advantages such as a reduction in manufacturing costs, a downsizing of the display device, an improvement in yield, and a reduction in through-put, are obtained by forming various circuits and elements on the same substrate.
The active matrix EL display device provides a switching element formed of a TFT (hereinafter referred to as a switching element) on each of pixels and activates a driver element that conducts a current control by the switching TFT, to thereby make an EL layer (speaking rigidly, a light emitting layer) emit light. For example, Japanese Patent Application Laid-open No. Hei 10-189252 discloses the EL display device.
As the active matrix EL display device, there are proposed two EL element structures depending on light radiating directions. One of those structures is that a light emitted from the EL element penetrates an opposing substrate and is then radiated so as to enter eyes of an observer. In this case, the observer can recognize an image from the opposing substrate side. The other structure is that a light emitted from the EL element penetrates an element substrate and is then radiated so as to enter eyes of the observer. In this case, the observer can recognize an image from the element substrate side.
In the former structure, the light from the outside penetrates the opposing substrate and is then irradiated onto the TFTs existing in gaps between the respective pixel electrodes, to thereby deteriorate the TFTs. However, because the light from the outside is not high in intensity, the deterioration of the TFTs is not large.
On the other hand, in the latter structure generally frequently employed, because the light emitted from the EL element penetrates the element substrate and is then radiated, the light emitted from the EL element is irradiated onto the TFTs, resulting in such a serious problem in that the TFTs are deteriorated.
Also, a storage capacitor is provided in the pixel and a high aperture ratio is demanded for the pixel from the viewpoint of the display performance. If the respective pixels have the high aperture ratio, the light application efficiency is improved, thereby being capable of achieving the power saving and the downsizing of the display device.
In recent years, the fine pixel size is developed, and a higher definition image is demanded. The fine pixel size increases an area of one pixel on which the TFT and the wirings are formed, to thereby reduce the pixel aperture ratio.
Under the above circumstances, in order to obtain the high aperture ratio of each pixel within the limit of a regular pixel size, it is essential to efficiently layout circuit elements necessary for the circuit structure of the pixel.
As described above, in order to realize the active matrix EL display device high in pixel aperture ratio with a small number of masks, an entirely novel pixel structure that has not existed up to now is demanded.